Phase displacement free-electron laser

ABSTRACT

A free-electron laser (FEL), which in a preferred embodiment comprises a relativistic electron beam generator; an optical cavity capable of storing a co-propagating optical beam; an interaction region; a phase displacement device disposed in the interaction region; and an output coupler interposed in the optical cavity through which a predetermined portion of the co-propagating optical beam exits as a coherent output optical beam. In certain embodiments, an optical beam spectral filter may be interposed in the optical cavity. FEL components and methods of use are also disclosed. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope of meaning of the claims.

RELATED APPLICATIONS

[0001] The present invention claims priority from U.S. ProvisionalApplication No. 60/271,872 filed Feb. 23, 2001 and U.S. ProvisionalApplication No. 60/271,873 filed Feb. 23, 2001.

FIELD OF THE INVENTION

[0002] The present invention relates to free-electron lasers.

BACKGROUND OF THE INVENTION

[0003] The following references are cited in this background to betterillustrate the background of the invention: Ref. 1 U.S. Pat. No.3,822,410 issued to Madey for “Stimulated Emission of Radiation in aPeriodically Deflected Electron Beam”; Ref. 2 J. M. J. Madey,“Stimulated Emission of Bremsstrahlung in a Periodic Magnetic Field,” J.Appl. Phys., vol. 42, pp. 1906-1913, 1971; Ref. 3 J. M. J. Madey, D. A.G. Deacon, L. R. Elias, and T. I. Smith, “An Approximate Technique forthe Integration of the Equations of Motion in a Free-Electron Laser,” IlNuovo Cimento, vol. 51B, pp. 53-69, 1979; Ref. 4 W. B. Colson, FreeElectron Laser Theory, Ph.D. Dissertation, Stanford, CA: Department ofPhysics, Stanford University, 1977; Ref. 5 N. M. Kroll, P. L. Morton,and M. N. Rosenbluth, “Free-Electron Lasers with Variable ParameterWigglers,” IEEE J. Quantum Electron., vol. QE-17, pp. 1436-1468, 1981;Ref. 6 ibid; Ref. 7 C. A. Brau, Free-Electron Lasers, Boston, MA:Academic Press, 1990; pp. 236-255; Ref. 8 op. cit., ref. 3; Ref. 9 op.cit., ref. 5; Ref. 10 op. cit., ref. 7; pp. 255-258; Ref. 11 O. K.Crisafulli, E. B. Szarmes, and J. M. J. Madey, “Use of Inverse-Taperingto Optimize Efficiency and Suppress Energy Spread in an rf-LinacFree-Electron Laser Oscillator,” IEEE J. Quantum Electron., vol. 37, pp.993-1007, 2001; Ref. 12 U.S. Pat. No. 4,641,103 issued to Madey et al.for “Microwave Electron Gun”; and Ref. 13 U.S. Pat. No. 5,130,994 issuedto Madey et al. for “Free-electron laser oscillator for simultaneousnarrow spectral resolution and fast-time resolution spectroscopy.”.

[0004] A free-electron laser is a device used to convert the kineticenergy of a beam of relativistic free electrons to electromagneticradiation in the wavelength region between the mm-wave region in radiospectrum and the extreme ultraviolet region at optical wavelengths (Ref.1). As is known in the art, free-electron lasers work by exploiting theinteraction between a beam of relativistic electrons moving through aspatially oscillating transverse magnetic field and a co-propagatingbeam of electromagnetic radiation (Ref. 2).

[0005] In the presence of a strong optical field whose phase matches thephase of the transverse oscillations induced by the transverse magneticfield, the electrons' trajectories in phase and energy are governed by apair of coupled equations which can be reduced to the pendulum equation(Ref. 3; Ref. 4). The existence of a series of fixed points and anassociated set of stable, closed, periodic orbits (“buckets”) in thephase space trajectories followed by the electrons in such a system hasled to the development of a range of methods for enhancement of thepower output that can be obtained from such devices (Ref. 5).

[0006] The principal method for enhancement of free-electron laser poweroutput and efficiency employed to date has been the deceleration of theelectrons circulating in the periodic orbits around the stable fixedpoints in their phase space trajectories by decreasing the period and/oramplitude of the spatially oscillating magnetic field as a function ofposition along the interaction region (Ref. 6; Ref. 7). This method hasyielded only limited improvements in power output, and has the furtherdisadvantage of failing to extract a significant amount of energy fromthe electrons moving along the open trajectories outside the region ofphase stability, leaving the electrons emerging from the interactiondistributed over a range of energies as large as 10% or more.Independent of the limited enhancement in power output attainable bythis method, the large energy spread induced by this method hascomplicated attempts to recover the residual kinetic energy of the spentelectrons, and made it impossible to operate more than one free-electronlaser at a time using a single beam of electrons.

[0007] The expense of the accelerator systems required to produce theelectron beams required for free-electron laser operation and theintense ionizing radiation produced by the spent electrons emerging fromthe interaction region if not decelerated have constituted majorpractical barriers to the further development and commercialization offree-electron lasers. The invention described herein overcomes thesebarriers by exploiting a different lasing mechanism, phase displacement(Ref. 8; Ref. 9; Ref. 10), in which the kinetic energy of electronsmoving through an interaction region defined by a spatially oscillatingtransverse magnetic field and a co-propagating beam of electromagneticradiation is converted to light by facilitating the deceleration of theelectrons to lower energies along the open trajectories outside andbetween the regions of phase stability (“buckets”). By permittingdeceleration of nearly all the electrons in the beam by nearly the sameincrement, this method provides greater laser power output thanpreviously attainable by converting a greater fraction of the electronsaverage energy to electromagnetic radiation, while dramatically reducingthe spread in energy induced by the interaction thereby simplifying thetransport and recovery of the residual kinetic energy of the spentelectron beam and reducing the risk of production of hazardous ionizingradiation during operation.

[0008] Accordingly, the invention described herein provides a means toenhance the power output, efficiency and flexibility of free-electronlasers while reducing their cost and complexity and the cost of theancillary radiation shielding required to insure operator and publicsafety. These improvements are important to currently established andexisting uses for free-electron lasers including laser surgery anddiagnostics, materials processing, spectroscopy and remote sensing,laser power beaming, and high power laser weapons systems.

[0009] Although certain aspects of the phase displacement lasingmechanism were anticipated in the earlier publications, the inventorswere the first to rigorously examine the operation of systems based onthis mechanism under the conditions prevailing in practical use in whichlasers must start from noise in the small signal regime, evolvenaturally to achieve the conditions required for effective conversion ofelectron kinetic energy to light, and stably maintain these parametersfor a useful interval of time (Ref. 11). In the course of this new andunanticipated investigation, the inventors were able to establish thatsuch systems, properly designed, could start from noise and evolvenaturally to a stable operating configuration compatible with enhancedpower output and reduced output energy spread. However, the inventorshave also discovered a new instability capable of disrupting laseroperation outside a specific range of operating conditions, and havefurther established that this instability may be suppressed by limitingthe growth of the spectral components of the optical field which mediatethe instability.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic view of a free-electron laser of the presentinvention;

[0011]FIG. 2 is a schematic view of a microtron configuration of amulticolor phase displacement FEL;

[0012]FIGS. 3a-3 c are schematic views of possible arrangements ofinteraction regions and optical cavities;

[0013]FIGS. 4a-4 e are schematic views of alternative embodiments ofphase displacement devices; and

[0014]FIG. 5 is a schematic view of a preferred embodiment of an opticalbeam spectral filter.

DETAILED DESCRIPTION OF VARIOUS EXEMPLARY EMBODIMENTS

[0015] As used herein, the following terms have the following meanings:“magnetic a device capable of creating a spatially oscillating fieldtransverse magnetic field of predetermined period and generator”amplitude, both of which may vary with position along an axis of themagnetic field generator. The magnetic field generator may be interposedin the path of the electron beam generated by an electron beam generatorwhereby exchange of energy between the electron beam and aco-propagating optical beam is facilitated by interaction between thetransverse electron velocity and the transverse optical electric field.By way of example and not limitation, a magnetic field generator may bea wiggler or undulator as these terms are understood by those ofordinary skill in the FEL arts. “interaction a region, possibly in avacuum, disposed longitudinally region” along the direction of theelectron beam in which the co-propagating electron and optical beams areoverlapped with a field generated by the magnetic field generator and/ora phase displacement device. In some embodiments, there may be one orseveral distinct interaction regions disposed along the electron beam,or one or several distinct interaction regions disposed within anoptical cavity. “separatrix” a closed trajectory in electronlongitudinal phase space (where phase space coordinates are defined bythe energy and longitudinal position of an electron in the electronbeam) which separates open-orbit electron trajectories from closed-orbitelectron trajectories in phase space. Separatrices are determined by themagnitude and spatial dependence of the magnetic field generated by themagnetic field generator together with the electric field of theco-propagating optical beam, independently of the electron beam.“resonant the energy of the stable points in electron longitudinalenergy” phase space. “synchrotron the frequency at which electrons,located on closed-orbit frequency” trajectories about the stable pointswithin the separatrices, undergo successive revolutions in phase space.“phase a device that is capable of increasing resonant energydisplacement relative to the energy of an electron in the electrondevice” beam during the interaction of the electron with the opticalbeam in the interaction region. In some embodiments, the phasedisplacement device could be incorporated in the structure of themagnetic field generator. In other embodiments, the phase displacementdevice could be a device which is distinct from the magnetic fieldgenerator. “small-signal refers to laser oscillation and is the periodof time after regime” initial generation of the laser beam when thestable fractional increase of laser power after each complete round tripin the optical cavity is substantially constant over successive roundtrips. By way of example and not limitation, the small-signal regime ischaracterized by electron energy perturbations which are substantiallysmaller than the height of the phase space separatrices. “small-signalthe fractional increase of laser power after a single pass gain” throughthe interaction region in the small-signal regime. “saturated refers tolaser oscillation and is the state of dynamic regime” equilibrium,established following the initial growth of the intensity of the opticalfield, in which the stable fractional increase of laser power after eachcomplete round trip in the optical cavity has decreased to substantiallynegligible proportions compared to the fractional increase of laserpower in the small-signal regime. By way of example and not limitation,the saturated regime is characterized by electron energy perturbationswhich are a substantial fraction of the height of the phase spaceseparatrices. “saturated the fractional increase of laser power after asingle pass gain” through the interaction region in the saturatedregime.

[0016] In the figures, a general reference to a device or element isindicated by a numeral, e.g. “X” and two or more specific instances ofthe device or element is indicated by a trailing letter, e.g. “Xa.” Byway of example, as used herein, an optical cavity is generally referredto by the numeral “30” while two or more optical cavities will bereferred to individually as “30 a,” “30 b,” and the like.

[0017] Referring now to FIG. 1, a free-electron laser of the presentinvention, generally referred to herein by the numeral “10” or by “FEL10,” comprises relativistic electron beam generator 20 which may bedisposed at least partially within optional housing 12 (not shown in thefigures) and which may further include subharmonic energy modulationcavity 27; electron beam transport system, generally referred to hereinby the numeral “41;” optical cavity 30 which is capable of storingco-propagating optical beam 32; interaction region 50 disposed inoptical cavity 30; phase displacement device 60 disposed about apredetermined portion of interaction region 50; optical beam spectralfilter 70 interposed in optical cavity 30 along a predetermined portionof optical axis 34; and output coupler 80 interposed in optical cavity30. A predetermined portion of co-propagating optical beam 32 exitsoptical cavity 30 via output coupler 80 to produce coherent outputoptical beam 33.

[0018] In certain contemplated embodiments, FEL 10 further compriseshousing 12 (not shown in the figures) capable of maintaining a vacuum.In a preferred embodiment, the vacuum will be sustained at a pressure ofat most 10⁻⁶ Torr. In certain currently considered embodiments, housing12 may be substantially permanently sealed once the desired vacuum isachieved or may further comprise vacuum port 14 (not shown in thefigures) to be connected to an external vacuum device (not shown in thefigures). Additionally, one or more components, e.g. relativisticelectron beam generator 20 and/or optical cavity 30, may be partiallydisposed within housing 12.

[0019] Relativistic electron beam generator 20 further comprises cathode22 and accelerator 26. Relativistic electron beam generator 20 iscapable of producing relativistic electron beam 42 by acceleratingelectrons emitted from cathode 22 through a DC potential gradient, oneor more microwave accelerating cavities, a microwave linear accelerator,a linear induction accelerator, a circular induction accelerator, or thelike, or a combination thereof, as these terms are understood by thoseof ordinary skill in the FEL arts. In a preferred embodiment, axis 44 ofrelativistic electron beam 42, generated at least in part byrelativistic electron beam generator 20, is aligned along optical axis34 of optical cavity 30.

[0020] Cathode 22 may comprise a thermionic cathode, a photoemissivecathode, a field-effect cathode, or the like, or a combination thereof.In certain embodiments, cathode 22 is capable of photo-assistedemission, by way of example and not limitation including photo-assistedfield-emission, photo-assisted thermionic emission, or the like, or acombination thereof.

[0021] Ultraviolet laser 21 may be disposed proximate and illuminatecathode 22, enhancing current density emitted by cathode 22.

[0022] Relativistic electron beam generator 20 may further comprisemicrowave electron gun 25 and accelerator 26.

[0023] Accelerator 26 may comprise a microwave linear accelerator(microwave linac) operating at a predetermined frequency. In embodimentsusing a microwave linac, relativistic electron beam generator 20 mayfurther comprise electron energy modulator 27 capable of modulatingelectron energy at a sub-harmonic of the operating frequency of themicrowave linac.

[0024] Referring now to FIG. 2, relativistic electron beam 42 producedby relativistic electron beam generator 20 may be recirculated throughmicrowave linear accelerator 26 in a microtron configuration, as thatterm will be familiar to those of ordinary skill in the FEL arts, whereoptical cavities 30 a,30 b,30 c enclose independent interaction regions50 a,50 b,50 c within a plurality of straight sections of the microtron.Subharmonic energy modulator cavities 27 and de-modulator cavities 72may be used to shift the energy of alternate electron bunches passingthrough the interaction regions 50. Boost microwave cavities 28 may beused to replace the electron energy converted to light prior tore-injection of the recirculating electron beam into accelerator 26.Orbit length adjuster 77 may be used to adjust the phase at which thespent electrons are injected into accelerator 26 following the lastinteraction region 50 c to provide for deceleration of the electrons onsubsequent passes through accelerator 26.

[0025] Referring back to FIG. 1, in certain currently contemplatedembodiments, accelerator 26 may further comprise an inductionaccelerator which may be a linear induction accelerator, a circularinduction accelerator, or the like, or a combination thereof. As isknown in the FEL arts, an induction accelerator allows for generation ofa pulsed relativistic electron beam 42.

[0026] In other contemplated embodiments, accelerator 26 may furthercomprise a high voltage DC accelerator, in which the required voltage isgenerated by a Van de Graaf generator, a Cockroft-Walton generator, aresonant transformer, or the like, or a combination thereof.

[0027] In other currently envisioned embodiments, accelerator 26 may becomprise a linear microwave accelerator, a recirculating linearmicrowave accelerator, or the like, or a combination thereof. For theseaccelerators 26, relativistic electron beam 42 produced is a bunchedrelativistic electron beam 42. As is also known in the art, whereaccelerator 26 comprises a microwave linear accelerator disposedproximate to relativistic electron beam 22, accelerator 26 may becapable of accelerating electrons to full energy in either a single passthrough the microwave linear accelerator, a series of multiple passesthrough the microwave linear accelerator, or a combination thereof.

[0028] In certain contemplated embodiments, the residual kinetic energyof relativistic electron beam 42 emerging from a last interaction region50, e.g. 50 b, is extracted as microwave energy by injecting the spentelectrons into a microwave accelerator 26 at a phase at which themicrowave field acts to decelerate the electrons. The energy extractedas microwave power can either be absorbed as heat in a resistivemicrowave load, or coupled to electron beam generator 20 to reduce themicrowave power required for operation of electron beam generator 20.

[0029] In certain embodiments, microwave accelerator 26 is used todecelerate relativistic electron beam 42 and may be an accelerator 26operated independently from electron beam generator 20.

[0030] In certain other embodiments, microwave accelerator cavity 72(FIG. 2) may be used to accelerate the electrons in electron beamgenerator 20. In these embodiments, the microwave accelerator cavity 27can also be used to decelerate spent electrons such as by adjusting thephase at which the spent electrons are injected into the cavity.

[0031] In other contemplated embodiments of the invention, the residualkinetic energy of relativistic electron beam 42 emerging from the lastinteraction region, e.g. 50 b, is extracted as DC or pulsed electricalpower by decelerating the electrons in a static or pulsed electric fieldproduced by a Van de Graaf or Cockroft-Walton generator, a high voltageresonant transformer, or a pulsed high voltage transformer. The highvoltage generator used to decelerate the spent electrons can either bethe same generator used in electron beam generator 20 in the case of DCor induction accelerators 26, or a generator specifically constructedfor the purpose and independent of electron beam generator 20. Theelectrical energy extracted from the spent electrons is available toreduce the net power required for acceleration of the electrons inelectron beam generator 20.

[0032] Electric field generator 29 may be disposed about cathode 22 toenhance the emitted thermionic current density of thermionic cathode 22and accelerating electrons to energies greater than 500 keV. Forphotoelectric cathode 22, electric field generator 29 may be disposedabout cathode 22 to enhance the emitted photoelectric current density.In a preferred embodiment having electric field generator 29, anelectric field generated will be at least 10 megavolts/meter with apreferred range of 100 to 150 megavolts/meter.

[0033] The electric field required for operation of the cathode may begenerated by a DC potential gradient, by a pulsed potential gradientgenerated by a pulsed high voltage transformer, by the oscillatingmicrowave field in a microwave cavity, or by a combination thereof. Thespatial dependence of the electric field required for operation of thecathode in each possible embodiment of the invention must be controlledby shaping the contour of the cathode and the nearby conducting surfacesto minimize electron beam emittance as has been established in the priorart.

[0034] Electron beam transport system 41 may be defined within a singlearea or within multiple areas of FEL 10. In FIG. 1, electron beamtransport system 41 is illustrated as multiple electron beam transportsystems 41, i.e. electron beam transport system 41 a which extends fromelectron beam generator 20 to first magnetic field generator 52 a,electron beam transport system 41 b which extends between magnetic fieldgenerators 52 a and 52 b, and electron beam transport system 41 c whichextends from magnetic field generator 52 b to an electron beam dump oran energy recovery system, generally referred to as 23.

[0035] Referring now additionally to FIGS. 3a-3 c, single independentmagnetic field generator 52 may be disposed in a single predeterminedoptical cavity 30 (e.g., 30 a in FIG. 3a). A plurality of independentmagnetic field generators 52 (shown as 52 b and 52 c in FIG. 3b) may bedisposed within a single optical cavity 30 (e.g., 30 b in FIG. 3b). Inother currently contemplated embodiments, a plurality of independentmagnetic field generators 52 (shown as 52 d, 52 e, and 52 f in FIG. 3c)may be disposed within a plurality of optical cavities 30 (e.g., 30 c,30 d, and 30 e in FIG. 3c).

[0036] Each interaction region 50 may further comprise one or moreindependent magnetic field generators, generally referred to by thenumeral “52” and more specifically by “52 a” or “52 b” in FIG. 1.

[0037] Referring now to FIGS. 4a-4 e, for certain embodiments in whichphase displacement device 60 is incorporated into the structure ofmagnetic field generator 52, magnetic field generator 52 comprises aplurality of pairs of magnets 55, each magnet pair 55 havingpredetermined properties. Magnets 55 may be permanent magnets,electromagnets, pulsed electromagnets, pulsed iron-free electromagnets,hybrid permanent/electromagnets, or the like, or a combination thereof.

[0038] Each of the plurality of pairs of magnets 55 are positionedsymmetrically about optical axis 34. The alternating poles of magnets55, shown in FIGS. 4a, 4 b, and 4 c as “N” and “S,” create a spatiallyoscillating magnetic field required for operation of FEL 10. Spacing ofthe pairs of magnets 55 along optical axis 34 may be selected toincrease the period of the magnetic field with respect to position alonga predetermined portion interaction region 50 such as to increase theresonant energy by a predetermined desired value (FIG. 4a). In apreferred embodiment, the increase in resonant energy attributable tothe increase in period is at least twice the height of the phase-spaceseperatrix around which electrons drift during the interaction of theelectrons with optical beam 32 and the magnetic field.

[0039] Further, the predetermined properties of magnet pairs 55 may beselected to produce a spatially oscillating magnetic field disposedabout optical axis 34 whose field amplitude increases monotonically withposition along an interaction region to increase the resonant energy(FIG. 4b). The predetermined properties of magnet pairs 55 a, 55 b,etc., comprise physical dimensions of each magnet 55, the magnetization,and periodicity. By way of example and not limitation, although shownarranged linearly in the plane with alternating poles, otherarrangements are possible, such as the use of a helically wound bifilarelectromagnet.

[0040] In alternative embodiments, phase displacement device 60 mayinduce phase displacement by using a decelerating electric fieldgenerated by one or more microwave cavities 61 disposed within theinteraction region 50 (FIG. 4c), using one or more induction coils 62disposed within the interaction region 50 (FIG. 4d), using alongitudinal electric field produced along optical axis 34 by imposing apotential gradient along the boundaries of the interaction region (FIG.4e), or the like, or a combination thereof.

[0041] In a preferred embodiment, phase displacement device 60 inducesphase displacement that is varied in time between the small-signalregime of laser oscillation and the saturated regime of laseroscillation. The time variation of the phase displacement may increasethe magnitude of phase displacement from a predetermined first value toa predetermined second value.

[0042] In some embodiments, phase displacement device 60 induces phasedisplacement that is varied in time between the initiation of laseroperation at turn-on in the small signal regime and the large signalregime characteristic of gain saturation at high power. The timevariation of phase displacement may increase the magnitude of phasedisplacement from a predetermined first value to a predetermined secondvalue to improve the small signal gain while retaining the phasedisplacement required for efficient operation in the strong signalregime at high power.

[0043] In certain alternate embodiments of the invention, the timevariation of the phase displacement induced by phase displacementgenerator 60 may be determined by a feedback or feedforward controlsystem to compensate for unavoidable variations in electron beam energy,current, bunch length bunch profile and optical cavity reflective anddissipative losses.

[0044] Referring back to FIG. 1 and additionally to FIG. 5, in certainembodiments, optical beam spectral filter 70 is capable of altering theamplitude and phase of the spectral components of co-propagating opticalbeam 32 and may comprise an intracavity etalon, a diffraction grating, adispersive device, or the like, or a combination thereof.

[0045] In certain embodiments, optical beam spectral filter 70 comprisesan optical filter whose transfer function is designed to reject orattenuate optical frequencies outside of a predetermined band offrequencies. In other contemplated embodiments, optical beam spectralfilter 70 comprises an optical filter whose group velocity dispersion isdesigned spoil cavity synchronism outside of a predetermined band offrequencies.

[0046] In a certain contemplated embodiment of optical filter 70, theoptical filter comprises two parallel, uncoated birefringent crystals 90a,90 b enclosing a variable-space vacuum gap 91 (FIG. 5). The primaryfilter function is provided by vacuum gap 91 which forms a Fabry-Perotetalon, and the birefringent crystals 90 a,90 b are employed to provideadditional spectral selectivity by altering the reflectance of eitherthe resonant or off-resonant wavelengths in vacuum gap 91. Thecharacteristics of this embodiment of the optical filter include angleof incidence 92 with respect to optical beam 32 (which angle equals theBrewster angle in the preferred embodiment), the size of vacuum gap 91,the direction of crystal c-axis 93 a,93 b, and crystal thickness 94. Ina preferred embodiment, these characteristics are chosen so that thebirefringent passband for each birefringent crystal 90 is equal to thepassband of vacuum gap 91.

[0047] In a currently envisioned embodiment, the passband of vacuum gap91 is chosen to appropriately suppress the spectral components whichmediate the coupled electron-optical beam instability that can disruptthe operation of FEL 10.

[0048] As depicted in FIG. 5, optical filter 70 may be rotated aboutsurface normal 95 to vary the degree of birefringence in crystals 90,and vacuum gap 91 may be tunable using piezoelectric gap tuning.

[0049] Referring back to FIG. 1, in certain embodiments of the inventionusing pulsed electron beams, such as accelerated by a microwave linearaccelerator, optical cavity 30 may further comprise an interferometricMichelson or Fox-Smith resonator configured to phase lock optical pulsesof optical beam 32.

[0050] Output coupler 80 may further comprise a dielectric plateinclined at a predetermined angle with respect to a predeterminedportion of optical beam 32, a partially transmissive optical cavitymirror, a diffraction grating, an acousto-optic grating, or the like, ora combination thereof.

[0051] The inventors have discovered that the deceleration of theelectrons via the phase displacement mechanism in FEL 10 can bedisrupted by an instability of the coupled relativistic electron beam 42and generated optical field. Accordingly, in a currently preferredembodiment of the invention, optical filter 70 may be present and usedto suppress this instability by limiting the growth of the spectralcomponents of the optical field which mediate the instability.

[0052] In a second preferred embodiment, the parameters of operation maybe specified to achieve free electron laser operation in a regime inwhich the instability is absent.

[0053] In operation of a currently preferred embodiment, FEL 10,comprising relativistic electron beam generator 20, electron beamtransport system 41, magnetic field generator 52, phase displacementdevice 60, optical cavity 30, intracavity optical filter 70 and outputcoupler 80, is operated to simultaneously achieve free electron laseroperation at high power, the efficient conversion of electron beamkinetic energy to light, and the uniform deceleration of the electronsmoving along the streamlines in phase space between the seperatrices. Inthis embodiment, a center frequency of optical filter 70 may first beset at a the intended wavelength for laser operation. Bandwidth ofoptical filter 70 may then be set at a first predetermined value equalto or less than the synchrotron frequency. Housing 12 may optionally bepart of FEL 10 as described above and may be evacuated to produce adesired vacuum.

[0054] The cavity outcoupling ratio may be set to a second predeterminedvalue equal to the difference between the saturated gain attainable atthe electron current available from electron beam generator and thereflective and dissipative and diffractive losses of optical cavity 30.Phase displacement device 60 may then be set to increase the resonantenergy relative to the electron beam energy over the length ofinteraction region 50 by a third predetermined value at least equal totwice the height of the seperatrix attributable to laser operation atthe value of optical power the system has been designed to operate.

[0055] The energy of relativistic electron beam 42 provided by electronbeam generator 20 and electron beam transport system 41 may be set at afourth predetermined value above the resonant energy at the entrance tothe interaction region 50 and differing from the resonant energy by theheight of the seperatrix attributable to laser operation at the value ofoptical power the system has been designed to operate.

[0056] In a preferred embodiment, relativistic electron beam 42 is thenbe left on for a time sufficient for the optical power in the cavity togrow from the small signal to the saturated regime, and for theadditional period of time for which the FEL 10 must supply laser powerin the application it has been designed to serve.

[0057] Exemplary operating parameters for operation of the currentlypreferred embodiment of FEL 10 are listed in Table 1. TABLE 1Specifications for the currently preferred embodiment of the invention.Parameter Definition Specification Optical beam parameters λ Opticalwavelength 1 μm δ_(c) Total cavity losses 5% Z_(R) Rayleigh range 40 cmP_(peak) Micropulse peak laser power 97 MW P_(ave) Macropulse averagelaser power 270 kW Electron beam parameters thermionic cathode microwavegun rf linac γ Electron beam energy/mc² 162.48 τ_(p) Bunch length 1 psQ_(b) Bunch charge 75 pC I_(peak) Micropulse peak current 75 A β_(X)Horizontal beta function 40 cm β_(y) Vertical beta function 57 cm ε_(X)Horizontal normalized emittance 10•mm•mrad ε_(y) Vertical normalizedemittance 5•mm•mrad T_(e) Gun repetition rate 2.856 GHz τ_(Ω) Macropulseduration 4.2 μs Wiggler parameters λ_(W) Wiggler magnet period 2.4 cm(fixed period) N_(W) Number of wiggler periods 83 L_(W) Wiggler length1.992 m {circumflex over (K)}² RMS vector potential squared 1.2 (atentrance) Field taper Linear increase of resonant 2.9 MeV energy

[0058] In a further preferred embodiment, FEL 10 may compriserelativistic electron beam generator 20, optical cavity 30, interactionregion 50, magnetic field generator 52, phase displacement device 60,and output coupler 80 as described herein above. Housing 12 mayoptionally be part of FEL 10. However, in this embodiment, opticalfilter 70 is not present. Instead, in this embodiment, laser parameters,including but not limited to peak current and/or output coupling, may bechosen to suppress the growth of the coupled electron-optical beaminstability.

[0059] FEL 10 may be operated by setting the cavity outcoupling ratio toa first predetermined value such that the total cavity losses are largeenough to prevent the growth of the spectral components which mediatethe coupled electron-optical beam instability that can disrupt theoperation of FEL 10. Phase displacement device 60 may then be adjustedto increase the resonant energy relative to the electron beam energy bya second predetermined value. This second predetermined value may beselected to be a value that is at least twice the height of theseperatrix formed in the course of laser operation at the value ofoptical power the system has been designed to operate.

[0060] The energy of electron beam 42 in this embodiment is set to athird predetermined value which is greater than the resonant energy atthe start of the interaction by an amount at least equal to the heightof the separatrix formed in the course of laser operation at the valueof laser power at which FEL 10 has been designed to operate.

[0061] Conditions for regenerative amplification, an initial extractionof energy from the electrons in electron beam 42, and a build-up ofoptical power within optical cavity 30 may then be established bysetting the electron beam current initially to a value sufficient toinsure that the small signal laser gain exceeds the sum of thereflective, dissipative, diffractive, and outcoupling losses in opticalcavity 30.

[0062] Electron beam 42 is maintained at this current for a period oftime sufficient for the optical power in optical cavity 30 to grow fromthe small signal to the saturated regime. The saturated gain may beadjusted by changing the electron beam current provided by electron beamgenerator 20 to maintain a saturated gain equal to the sum of the cavityoutcoupling fraction and the reflective and dissipative and diffractivecavity losses at the specified value of laser output power. Relativisticelectron beam 42 may then be left on for the additional period of timefor which the FEL 10 must supply laser power in the application it hasbeen designed to serve.

[0063] Exemplary operating parameters for operation of this alternateembodiment of FEL 10 are listed in Table 2. TABLE 2 Specifications forthe second preferred embodiment of the invention. Parameter DefinitionSpecification Optical beam parameters λ Optical wavelength 1 μm δ_(c)Total cavity losses 5% Z_(R) Rayleigh range 40 cm P_(peak) Micropulsepeak laser power 26 MW P_(ave) Macropulse average laser power 220 kWElectron beam parameters thermionic cathode microwave gun rf linac γElectron beam energy/mc² 162.48 τ_(p) Bunch length 3 ps Q_(b) Bunchcharge 75 pC I_(peak) Micropulse peak current 25 A β_(X) Horizontal betafunction 40 cm β_(y) Vertical beta function 57 cm ε_(X) Horizontalnormalized emittance 10•mm•mrad ε_(y) Vertical normalized emittance5•mm•mrad T_(e) Gun repetition rate 2.856 GHz τ_(Ω) Macropulse duration4.2 μs Wiggler parameters λ_(W) Wiggler magnet period 2.4 cm (fixedperiod) N_(W) Number of wiggler periods 83 L_(W) Wiggler length 1.992 m{circumflex over (K)}² RMS vector potential squared 1.2 (at entrance)Field taper Linear increase of resonant 2.9 MeV energy

[0064] FEL 10 may be operated in a multicolor phase displacement mode.In this mode, FEL 10 further comprises two or more independent magneticfield generators 52 in a single optical cavity 30 (FIG. 3a); two or moreindependent optical cavities 30 (each with one or more independentmagnetic field generators 52) using a common electron beam generator 20(FIGS. 3b, 3 c); or two or more independent optical cavities 30 (eachwith one or more independent magnetic field generators 52) using acommon recirculating electron beam generator 20 (FIG. 2).

[0065] In an alternate multicolor embodiment, the energy of electronbeam 42 may be modulated using a single subharmonic energy-shiftingcavity 27 with a single magnetic field generator 52 and optical cavity30.

[0066] FEL 10 may also be operated in an energy recovery phasedisplacement mode using an in-line decelerating system (DC, induction,or microwave) distinct from electron beam generator 20 and/or energyrecovery using recirculation through a common set ofaccelerating/decelerating microwave cavities in electron beam generator20.

[0067] While the present invention has been described above in terms ofspecific examples, it is to be understood that the invention is notintended to be confined or limited to the examples disclosed herein. Onthe contrary, the present invention is intended to cover variousstructures and modifications thereof included within the spirit andscope of the appended claims.

We claim: 1) A free-electron laser (FEL) comprising: a. a relativisticelectron beam generator further comprising a cathode and an accelerator;b. an optical cavity capable of storing a co-propagating optical beam inwhich an axis of a relativistic electron beam generated at least in partby the relativistic electron beam generator is aligned along an opticalaxis of the optical cavity; c. an interaction region disposed in theoptical cavity at least partially about the optical axis, theinteraction region further comprising a magnetic field generator; d. aphase displacement device disposed about a predetermined portion of theinteraction region at least partially about the optical axis; e. anoptical beam spectral filter interposed in the optical cavity along apredetermined portion of the optical axis; and f. an output couplerinterposed in the optical cavity in communication with a co-propagatingoptical beam coincident with the optical axis; g. wherein apredetermined portion of a co-propagating optical beam exits the opticalcavity via the output coupler to produce a coherent output optical beam.2) The FEL of claim 1 further comprising a housing capable ofmaintaining a vacuum wherein the housing is capable of sustaining avacuum pressure of at least 10⁻⁶ Torr. 3) The FEL of claim 1 furthercomprising a plurality of interaction regions, each interaction regionfurther comprising at least one independent wiggler disposed in a singlepredetermined optical cavity. 4) The FEL of claim 1 wherein the magneticfield generator further comprises a phase displacement device. 5) TheFEL of claim 4 wherein the phase displacement device induces phasedisplacement by using an inverse-taper of a magnetic field generated bythe magnetic field generator. 6) The FEL of claim 1 wherein phasedisplacement device induces phase displacement that is varied in timebetween a small-signal regime of laser oscillation and a saturatedregime of laser oscillation. 7) The FEL of claim 6 wherein the timevariation of the phase displacement increases the magnitude of phasedisplacement from a predetermined first value to a predetermined secondvalue. 8) The FEL of claim 1 wherein the optical beam spectral filter iscapable of narrowing an envelope of an optical spectrum of theco-propagating optical beam and further comprises at least one of anintracavity etalon, a diffraction grating, or a dispersive device. 9)The FEL of claim 1 wherein the optical beam spectral filter is capableof suppressing an axial cavity mode of an optical spectrum of theco-propagating optical beam and further comprises at least one of anintracavity Michelson-mirror interferometer or an intracavity Fox-Smithinterferometer. 10) The FEL of claim 1, wherein the magnetic fieldgenerator further comprises a plurality of pairs of magnets havingpredetermined properties, each of the plurality of pairs of magnetspositioned symmetrically about the optical axis of the optical cavitywherein a periodic magnetic field required for FEL operation isgenerated by the plurality of pairs of magnets. 11) The FEL of claim 10wherein a spacing of the pair of magnets along the optical axis of theoptical cavity is selected to increase the period of the magnetic filedmonotonically with a position along a predetermined interaction regionto increase resonant energy by a value at least twice the height of thephase-space seperatrix around which electrons drift during theinteraction of the electrons with the optical beam and the magneticfield. 12) The FEL of claim 1 wherein the output coupler furthercomprises at least one of: a. a dielectric plate inclined at apredetermined angle with respect to the predetermined portion of theoptical beam; b. a partially transmissive optical cavity mirror; c. adiffraction grating; or d. an acousto-optic grating. 13) A free-electronlaser (FEL) comprising: a. a relativistic electron beam generator; b. anoptical cavity capable of storing a co-propagating optical beam in whichan axis of a relativistic electron beam generated at least in part bythe relativistic electron beam generator is aligned along an opticalaxis of the optical cavity; c. an interaction region disposed in theoptical cavity, the interaction region further comprising a magneticfield generator; d. a phase displacement device disposed about apredetermined portion of the interaction region, and e. an outputcoupler interposed in the optical cavity to induce a predetermined valueof round-trip cavity losses; f. wherein i. cavity losses at saturationare limited to a predetermined value sufficient to allow uniformdeceleration of electrons along the streamlines between theseparatrices; and ii. round-trip cavity losses at saturation have amagnitude sufficient to prevent development of coupled electron-opticalbeam instability. 14) A method of operating a general phase-displacementFEL oscillator, for an FEL comprising a relativistic electron beamgenerator, an electron beam transport system, a magnetic fieldgenerator, a phase displacement device, an optical cavity, and an outputcoupler, the method comprising: a. setting a cavity outcoupling ratio toa first predetermined value; b. adjusting the phase displacement deviceto increase the resonant energy relative to the electron beam energy bya second predetermined value; c. creating the electron beam; d.accelerating the electron beam in the relativistic electron beamgenerator; e. directing the accelerated electron beam to the interactionregion by the electron beam transport system; f. injecting the directedelectron beam into the interaction region by the electron beam transportsystem; g. setting the electron beam energy to establish the energy ofthe electrons provided by the electron beam generator and electron beamtransport system at a third predetermined value; h. establishingconditions for regenerative amplification, an initial extraction ofenergy from the electrons in the beam, and a build-up of optical powerwithin the optical cavity by setting the electron beam current initiallyto a value sufficient to insure that the small signal laser gain exceedsthe sum of the optical cavity reflective, dissipative, and outcouplinglosses; i. leaving the electron beam on for a period of time sufficientfor the optical power in the cavity to grow from the small signal to thesaturated regime; j. configurably adjusting the large signal laser gainby changing the electron beam current provided by the electron beamgenerator to maintain the large signal gain at a fourth predeterminedvalue k. whereby laser operation at high power, efficient conversion ofelectron beam energy to light, and uniform deceleration of all theelectrons in the electron beam are achieved simultaneously. 15) Themethod of claim 14 wherein the second predetermined value in step (b) isat least twice the height of the seperatrix formed in the course oflaser operation at the value of optical power the system has beendesigned to operate. 16) The method of claim 14 wherein the thirdpredetermined value of step (g) is below the resonant energy at thespecified operating wavelength and differing from the resonant energy bythe height of the seperatrix formed in the course of laser operation atthe value of laser power the system has been designed to operate. 17)The method of claim 14 wherein the fourth predetermined value of step(j) is equal to the sum of the cavity outcoupling fraction and thereflective and dissipative cavity losses at the specified value of laserpower output.